While Google Assistant is rolling out to more devices, Microsoft is continuing to improve its own virtual helper. Microsoft's Cortana assistant made the jump to Android in 2015, and has continued to improve since with regular updates. The latest update, version 2.5, allows users to add Cortana to the lockscreen.
Once enabled in the settings, Cortana will add a small floating button to the lock screen. Swiping left or right on the button brings up Cortana's information feed, containing news, upcoming appointments, and other helpful information. However, there is no way to actually ask Cortana anything - presumably due to security concerns.
Cortana 2.5 should be live for everyone on the Play Store, but if not, you can grab it from APKMirror.